I Just bought a Huge Hibiscus in a Pot

It may have been outside before. I have it indoors and it is turning yellow on the leaves and I suspect spider mites. I sprayed as much as I could with something from Miracle Grow. Leaves falling like crazy. I know it is stressed but it is 7-8 feet tall and I can't wash it down. Should I spray it with soapy water or can I paint the leaves with a cooking oil or some other natural method. I can't move it far. I bet it weighs 100 pounds. Help!